3-Phase Motor Starter with Overload Protection: Sizing Guide

“3-phase motor starter with overload protection” is one of the most-searched sourcing phrases in motor control — and one of the most misunderstood. A starter is not a contactor with a bigger label; it is a tested chain of five parts whose ratings must match each other and the motor nameplate. Size any link from the wrong number and the failure is silent: the breaker rides through the overload, the motor cooks, and nobody trips. This page walks the four-step sizing chain with worked numbers, then lists the five mistakes that survive commissioning.

Quick Answer: What’s Inside a Starter

The key division of labor: the SCPD clears short circuits, the overload relay clears overloads. A breaker sized to survive inrush (up to 250 % of FLA) will happily let a 150 % overload cook a motor for minutes — the overload contact stops that by interrupting the coil circuit, not the power circuit. That is why the contactor-in-a-starter role in our contactor selection guide is only one link of the chain.

Step 1: Read the Nameplate, Not the kW

Everything downstream starts from the motor’s nameplate full-load amps (FLA) at the voltage and connection actually wired. Two traps:

• NEC tables vs nameplate. NEC Tables 430.248/430.250 values drive wire sizing and short-circuit protection, but NEC 430.6(A)(2) points overload settings at the nameplate. A modern premium-efficiency motor often draws 10–15 % less than the table value; setting the relay from the table silently loosens protection.
• Dual-voltage and multi-speed motors. Use the FLA column for the connection in use — the low-voltage column on a high-voltage-wired motor roughly doubles the protection level.

Step 2: The Contactor — AC-3 at the Real Voltage

Select the contactor so its AC-3 rated operational current at your voltage meets or exceeds the FLA, with duty and ambient checked on top. The mechanics of reading Ie tables are in our AC-3 rating chart guide; the utilization-category logic behind AC-3 is in the AC-1 to AC-4 explainer. Two conditions push the frame up from the paper answer: plugging/jogging duty (AC-4 territory, see the AC-3 vs AC-4 comparison) and hot enclosures — a 45 °C panel already eats 5–10 % of the AC-3 rating.

Step 3: The Overload Relay — Set and Class

Two decisions here: the dial setting and the trip class.

Setting. IEC practice is to set the dial at nameplate FLA and let the relay’s curve provide the margin. NEC 430.32(A)(1) caps the setting: 125 % of FLA for motors with marked service factor ≥ 1.15 or marked temperature rise ≤ 40 °C, otherwise 115 % — with 140 %/130 % escape values under 430.32(C) only if the motor cannot start at the lower setting. The dial discipline and nuisance-trip causes are covered in our overload field setting guide.

Trip class. The class number is the trip time at 7.2× setting under IEC 60947-4-1: Class 10 trips within 10 s (pumps, fans), Class 20 within 20 s (high-inertia loads), Class 30 within 30 s (long accelerations). Match the class to the measured start time — the thermal-damage curve logic is in our motor thermal damage vs relay trip curve guide.

Step 4: Short-Circuit Coordination and the Enclosure

The contactor and overload must be protected by an SCPD rated for the available fault current, and the pair must be a tested combination:

• Type 1 / Type 2 (IEC 60947-4-1). Type 1: after a short circuit, the starter may be damaged but must be safe. Type 2: the starter must resume service, at most with contact cleaning — the specification buyers actually want.
• SCCR (UL/NEMA markets). The assembled starter’s short-circuit current rating must meet or exceed the available fault current at its terminals.
• Don’t mix brands. An untested contactor + relay + fuse trio has no coordination rating at all, whatever the individual datasheets claim.

When the disconnect, SCPD, contactor and overload ship in one certified box, that is a combination starter — the architecture options are compared in our combination vs non-combination starter guide.

Worked Examples

Both examples assume 4-pole induction motors at nameplate; a hot enclosure or frequent starting pushes both selections up a frame — the interactions are the same ones our duty cycle guide quantifies. For the switching component itself, the CJX2 series 3-phase AC contactor covers the 9–95 A starter range, with the wider frames in the AC contactor category.

Five Mistakes That Survive Commissioning

Special Starter Types: Reversing, Star-Delta and Part-Winding

The four-step sizing holds for a plain DOL starter, but reduced-voltage and reversing schemes redistribute the currents, and each has one sizing trap:

The star-delta row hides the most common field error: the overload relay is set to nameplate FLA even though it sits in a path that only ever sees winding current (or the reverse, depending on layout). Follow the manufacturer’s wiring diagram for where the relay senses, then set the dial to the current that path actually carries — the nameplate current is the anchor, not always the dial value. The coil-side implications of switching two contactors in sequence (control wiring, feedback, dead time) are the same ones covered in our contactor wiring guide.

الأسئلة الشائعة

Do I need both a breaker and an overload relay?

Yes, for any motor that can overload. The breaker or fuses clear short circuits; the overload relay is the only device protecting the winding from a slow overload. The combination is the defining feature of a motor starter.

What size contactor for a 15 kW motor?

At 400 V a 15 kW 4-pole motor draws roughly 27–29 A on the nameplate, so a 32 A AC-3 frame is the normal answer — but confirm from the plate and re-check for plugging duty, hot enclosures and altitude, any of which can push the selection to 40 A.

Which trip class should I choose?

Class 10 covers most pumps, fans and machine tools that reach speed in a few seconds. Choose Class 20 for loaded conveyors, large fans and compressors with starts beyond 10 s, Class 30 for extreme inertia. Verify with a measured start time at 7.2× current rather than assuming.